The signs that appear adjacent to airport runways are subject to particular government regulations. For instance, there are regulations regarding the size of the signs and particular characteristics of the light emitted by the sign. In particular, there are regulations regarding the homogeneity of the emitted light, so that “hot spots” in the sign brightness are reduced or eliminated. These regulations are published by the Federal Aviation Administration, and an example of a specification regarding these signs is a published document with the name “FAA AC 150/5345-44J”. An example of a requirement from this document is that particular styles of runway sign “must have an average luminance of 10 to 30 foot lamberts (fL)”.
A typical two-sided runway sign is lit from its interior. Typically, the sign has two fluorescent bulbs or two remote phosphor LED lamps of the type shown in U.S. Pat. No. 7,618,157, one along its top edge and one along its bottom edge. For simplicity, the remainder of this document refers to fluorescent lamps, since they have been predominantly used in the field. The light emitted from these lamps has a relatively broad distribution in angle, and the sign relies on one or more diffusers within the interior of the sign to redistribute the light so that the hot spots nearest the lamps are sufficiently dampened. The diffusers randomize and redistribute the light in order to make the sign appear more uniform in brightness. These existing fluorescent lamps are typically connected electrically and supported mechanically by conventional tombstone-style connectors. Conventional runway signs are depicted in U.S. Pat. No. 8,099,886 (Ellerton), U.S. Pat. No. 7,377,062 (Ellerton), and U.S. Pat. No. 6,946,975 (Hansler et al.).
In recent years, there has been an effort to retrofit existing runway signs with new interior optics that use light emitting diodes (LEDs) as their light sources, which use less energy and have longer lifetimes than their fluorescent counterparts. See, for example, U.S. Pat. No. 7,618,157 (Galvez et al). The lamp discussed in Galvez uses blue LEDs that emit light toward a remote phosphor, which is located away from the blue LEDs on a cover of the lamp. The phosphor absorbs a portion of the blue light and emits yellow or yellow-green light, so that the combination of the non-absorbed blue light and the emitted yellow or yellow-green light appears white. Note that the phosphor shown in FIGS. 1 and 2 of Galvez absorbs the incident blue light from any incident direction, but radiates the yellow light or yellow-green light isotropically into all directions.
Other known LED-based light elements and sub-systems are discussed in FIGS. 1 and 2 of U.S. Patent Application Publication No. 2009/0261706 (Sorella et al.), FIGS. 3 and 8 of U.S. Pat. No. 7,461,951 (Chou et al.), FIGS. 40a and 40d of U.S. Patent Application Publication No. 2003/0002179 (Roberts et al.), U.S. Patent Application Publication No. 2012/0155095 (Ryan Jr.), U.S. Pat. No. 7,572,029 (Tsai), and U.S. Pat. No. 6,296,376 (Kondo).